#2. Low ratings

During WWE Q1 2020 financial call, Vince McMahon blamed RAW's dipping ratings on Brock Lesnar's absence and establishing new talent:
“It takes a lot to get these new talent over. We no longer have Brock Lesnar anymore, obviously.”
How Brock Lesnar ruined another wrestler's career - Watch Now!
While it might be hard to accept, there is some truth to it. Brock Lesnar brings the ratings and post-WrestleMania, things can dip at times. Moreover, given the circumstances, there seems to be a dip in interest in many aspects of entertainment, not just pro wrestling and WWE.
One way to ensure that they can get those ratings back is by bringing back a big name such as AJ Styles. However, to do that, they need to start making him a focal point of WWE programming again. Given that he's almost always been a main eventer in WWE, it wouldn't be surprising to see him in a big spot again.
In the eyes of many, he would be the perfect challenger to Drew McIntyre, especially since he's a heel and not a babyface.
A top WWE star is missing in action